A construction machine such as a hydraulic excavator in the background art is generally provided with hydraulic actuators such as hydraulic cylinders and hydraulic motors serving as actuators for driving movable portions in respective portions of the machine, and provided with an engine serving as a drive source for a hydraulic pump as a hydraulic source. Patent Document 1 has disclosed an operating device applied to a construction machine of this type. In the operating device, a required number of pilot type directional control valves are disposed between the hydraulic pump and the respective hydraulic actuators, and a pilot valve for supplying pilot hydraulic pressure to a pilot port of each directional control value in accordance with an operation amount of an operation lever operated by the operator is provided so that the pilot hydraulic pressure derived from the pilot valve is supplied to the pilot port of a predetermined one of the directional control valves corresponding to the operated operation lever, so as to change over the direction control valve and drive the hydraulic actuator corresponding to the directional control valve.
In addition, in recent years, for example, as described in Patent Document 2, there has been proposed a hybrid construction machine which uses hydraulic actuators and an electric motor together as actuators for driving respective portions of the machine, and which uses an engine and a power generator motor together as a source for driving a hydraulic pump, so that the fuel consumption of the engine can be improved and the exhaust gas amount and the noise level can be reduced. In most hybrid construction machines in the background art, hydraulic actuators are driven for excavation and traveling while an electric motor is used for performing operation for swinging a swing structure (for example, upperstructure in a hydraulic excavator).
Patent Document 2 has disclosed a technique in which both an operating unit for driving hydraulic actuators and an operating unit for driving an electric motor are constituted by hydraulic pilot operated valves, and pilot hydraulic pressure derived from each hydraulic pilot operated valve is converted into an electric signal by a pressure sensor and outputted to a control portion, from which a control signal for the electric motor is outputted, so that a feeling of operation on the operating unit operated by an operator when the hydraulic actuators are driven and a feeling of operation on the operating unit operated by the operator when the electric motor is driven can be standardized to cancel a feeling of strangeness given to the operator. In the case where such a control system for the electric motor is used, normal control for swinging a vehicle body is difficult when an abnormality occurs in any one of the pressure sensors, the control portion and the electric motor. Thus, it is likely that there may arise a disadvantage that the vehicle body swings with a velocity or a direction not intended by the operator. In order to avoid this disadvantage, Patent Document 2 also has disclosed a technique in which the pressure sensors for detecting the operation amounts of the hydraulic pilot operated valves are made redundant so that electric signals outputted from the respective pressure sensors can be compared in the control portion so as to stop the electric motor properly when an abnormality is detected.